Conventional horizontal axis wind turbine (HAWT) systems consist basically of rotor blades, a turbine rotor shaft, a nacelle containing a single generator to which the turbine rotor is connected directly or indirectly using speed changing gear boxes and cooling system, and a tower supporting the nacelle. This conventional HAWT system architecture is subject to three major constraints.
A first major constraint of conventional HAWT systems is the requirement to constantly keep track of the delivery of electrical power generated from the generator to the ground. Specifically, the nacelle comprising a generator should rotate, or yaw, continuously to follow an unpredictably varying wind direction. This is conventionally accomplished either by using passive yawing mechanisms, such as the tail vane in small size HWAT systems, or by activating a motorized yawing mechanism, as in medium and large scale systems.
In small sized HAWT systems, the electrical power generated from the generator contained in the nacelle is conventionally transmitted by using slip-ring and brush mechanisms. Despite electrical energy loss and maintenance problems, this mechanism may be a practical as well as an economical method for small HAWT systems, because the amount of transmitted current is generally rather small, in the order of a few tens of amperes. However, in larger HAWT systems, such as those generating megawatts of power, the current required to be transmitted may be in the order of thousands of amperes. In these large HAWT systems, the brush mechanism may not be a viable method, because of the large amount of expected energy loss caused by unstable contact resistance between the brush and the slip-ring.
The method conventionally employed in large HAWT systems for the same purpose involves a direct connection by electric cable from the generator to the ground, with enough slack to allow the nacelle to turn through predetermined angles in two directions. When it is detected that the predefined angle limit is reached, by closely monitoring the amount of cumulative angle the nacelle yaws in either direction, the system is shut down and moved into unwinding mode.
To address the electrical energy transmission problem of HAWT systems, vertical axis wind turbine (VAWT) systems, offering an alternative wind turbine architecture, are conventionally used. In a VAWT system, the generator is fixed to the ground, thereby eliminating the energy transmission problem. However, a main drawback of VAWT systems comes from the fact that their efficiency is generally lower than those of comparable HAWT systems. Furthermore, large-capacity VAWT systems are often difficult to build.
A second major constraint of conventional HAWT systems comes from the structure in which the nacelle carries almost every apparatus necessary for power generation, including rotor blades, generators, gearboxes, cooling systems, and inverter-converter systems. Due to space restrictions in conventional architectures, it is generally difficult to attempt more efficient and economic layouts of apparatuses. For example, a structure implementing a single large-size generator and gearbox typically requires a large cooling system, because the heat density, i.e., heat generated per unit space, is generally too high for natural cooling, due to the highly congested layout of system components within a restricted space.
Further, a maintenance problem may remain. When a large and heavy component, such as a generator, needs to be replaced, conventional HAWT systems typically require a large, costly tower crane to be called to the site of installation.
A third major constraint of conventional HAWT systems relates to size expansion limits when attempting to increase the power generated by a single installation site, not only from a design viewpoint but also from an economic viewpoint. As the size of the moving part, especially the rotor blades, increases in length to sweep larger area, the swept area and the cross sectional area of the blade increase as a square of the length, while the mass of the blade increases as a cube of the length. Thus, the mass of the blade increases faster than the swept area, and the energy produced per unit mass of moving part decreases. Thus, conventional large-scale HAWT systems are often criticized, in favor of small sized systems. Eventually, apart from the increased costs, conventional large-scale HAWT systems present mechanical design and construction problems, as well as the logistical problem of transporting large-sized objects from a factory to a construction site.
Basic structures using bevel gear system are described in “Wind turbine-generator,” U.S. Pat. No. 4,291,233, filed Jan. 29, 1980 (Reference [1]), “Multi-unit rotor blade system integrated wind turbine,” U.S. Pat. No. 5,876,181, filed Jun. 23, 1995 (Reference [2]), and “Over-drive gear device,” U.S. Pat. No. 5,222,924, filed Sep. 8, 1992 (Reference [3]); References [1], [2] and [3] are herein incorporated by reference. In Reference [1], by employing gearing of three bevel gears, the horizontal rotation of a turbine rotor may be translated into two vertically counter-rotating motions, which may be delivered respectively to the rotor and field parts of a single generator, thereby accomplishing increased relative rotation speed between the stator and the field parts of the generator. In this case, however, the use of slip-ring and brush mechanism may be inevitable and it may be impossible to realize dualization.
In Reference [2] a bevel gear system is proposed, which may allow for the collection of more wind energy from multiple wind turbine rotors installed at multiple positions on a single tower, while in Reference [3], the planetary gear may be used to increase the rotational speed applied to the axis of a generator. In both cases, the destination of the collected wind energy may be the single generator fixed to the tower, which may be very favorable from the view point of energy transmission. But there may be a drawback in this embodiment. When the collected wind energy is fed into the shaft of the single generator fixed to the tower, the reaction torque counteracting the generating torque, equal in magnitude but opposite in direction, may be applied to the nacelle. This implies that without any breaking system or active yaw control system, the nacelle may no longer be able to face the wind. In other words, the proposed system may operate in such a way that a torque harvested from the wind is applied to the generator, while the same amount of torque should be applied to the nacelle inevitably to prevent the nacelle from yawing away from wind. Hence, this system may make it not only impossible to build a free yawing HAWT system, but also impractical to build HAWT system even with a motorized yawing control system, due to the unnecessarily severe burden passed to the yawing control system in maintaining yaw direction of nacelle, which may result in wasting valuable energy and internally building up superfluous mechanical stress as well.
When attempting to convert horizontal rotation to vertical rotation through bevel gearing, it may be impossible by using a single generator rigidly fixed to the tower to eliminate the yawing torque applied to the nacelle created in reaction to the torque the induced at generator.